Fix board message clipping and color code behavior.

[AliceLR]

• The board message now clips at the correct position when it overflows the right edge of the board. Previously, it would always be clipped at BYTE 80 instead of after the final byte that would display on screen. This caused wrapping bugs with non-centered message columns >0 and would clip messages containing color codes too early.
• When the board message clips the right side of the screen, the next line displayed will now use the correct color code from the previous line (as if no clipping had occurred) instead of whatever the last color that displayed on the screen was.
• Fixed some minor errors in color_string_length and color_string_index_of. They should now behave identically to the old strlencolor with regards to color codes and escapes. They also now take const string pointers.